The present invention relates to an apparatus for removing solid particles from gases. In many industrial processes, particularly elevated or high temperature, high pressure procedures, gaseous streams are obtained containing solid particles which must be removed before the gaseous streams may be used, treated further or released. For example the ordinary combustion of coal produces an effluent gas containing flyash particles which cannot be released into the atmosphere. Similarly, the gasification or partial oxidation of coal often produces a product gas stream which, after bulk removal of solids, contains from 0.1 percent to 4.0 percent by weight, based on the total weight of the gas and solids, of various solid particles which comprise flyslag or flyash and, in some cases, unburned coal. Fluid catalytic cracking (FCC) processes also produce gaseous streams containing catalyst fines which cannot be allowed to contaminate the environment. Other processes which make use of apparatuses to clean gas are Pressurised Fluid Bed Combuster (PFBC) and Intergrated Gasification Combined Cycle (IGCC) coal based power systems.
When filtering hot gases at high temperatures filter elements of a ceramic type of many different embodiments are used today. In one embodiment filters may be composed of a large number of filter elements in a large filter container. This container is provided at its upper part with an outlet for cleaned gases and at its lower part with a collection space for solids separated in the filter and fallen down from the filter elements. Uncleaned gas enters the space below the filter elements at the lower side of the container. The filter elements may be suspended from a plate, provided with openings, in the upper part of the container where the plate supports the individual filter elements. Each opening in the plate is equipped with a filter element suspended below the opening. The filter element, sometimes referred to as filter candles, cleans the gases which flow through the filter element and further out through that opening in the plate which is associated with the respective filter element. The gas above the plate is thus freed from solids and brought to the outlet of the filter container. Solids from the uncleaned gas will adhere to the outside of the different filter elements. These solids will loosen when a gas is brought to pass through the filter elements in reverse direction during periodically recurring cleanings of the filter element, also referred to as backflushing. The solids from the collection space can be discharged at the lower part of the container.
Ceramic materials included in the filter elements of the kind described above have a stochastic composition from the point of view of strength, which is caused by the fact that in each produced ceramic component imperfections exist in the internal structure of the ceramic. This may lead to breakage of the component if it is subjected to stress, for example in the form of temperature transients.
In a filter with many filter elements, sometimes up to 800, there is always a risk that one or more filter elements will break. The consequence of this breakage of the filter element is that a certain part of the gas, that part which passes through the broken filter element, is admitted through this element in uncleaned state. In some applications the loss in separation efficiency due to the fact that only one or a small number of the filter elements break could be acceptable. For example a dust loading of between 2000-20000 ppm at the time of filter failure can be tolerated. The problem however is that solid particles present above the plate, as a result of the failure of one of the filter elements, will enter the downstream or clean end of the non-broken filter elements. Solids will enter the downstream or clean side of the non-broken filter elements due to entrainment the solids by the backflush gas. The solids will foul the clean side of the non-broken filter elements causing them to plug, resulting in that the whole filtering apparatus or filter bank becomes ineffective as filtering means.
Various apparatuses have been described which aim at ensuring that no solids will pass the broken filter elements or a group of elements comprising one or more broken elements. Such apparatuses are for example described in EP-A-551951 and EP-A-642377. No apparatuses have been described which allow some solids to pass the filter container as a whole and simultaneously ensure that the majority of the non-broken filter elements are not plugged by the solids which pass the broken filter elements. The present invention aims at providing such an apparatus.
The present invention provides an apparatus for filtering solid particles from a fluid containing solid particles, comprising a filter container, a plate provided with openings across the filter container such that the plate divides the container in an upper and lower space and a number of filter elements extending under the openings, wherein a plurality of outlet conduits are present which are in fluid communication with one or more filter elements, said outlet conduits having an outlet opening which is in fluid communication with the upper space and wherein around said outlet conduit a cap is present, which cap is provided with means for supplying a backflushing fluid for removing solid particles from said filter elements and wherein the volume between the cap and the outlet conduit is sufficiently large to ensure that predominately only the clean gas between cap and outlet conduit is entrained by the backflushing fluid into the filter element when removing solid particles.
The present apparatus provides a robust and simple design for filtering solids wherein the pressure drop of the apparatus is low. A further advantage is that the apparatus can be easily integrated into existing filter system designs in terms of seals, gaskets, fixtures and assemblies. Breakage of one or more filter element will not result in that all filter elements become plugged and cease to function as a filtering device. In normal operation the volume between the cap and the outlet conduit will be occupied by the cleaned gas leaving the filter element. This will also be the situation for the caps of the non-broken filter elements when one or more other filter elements break and cease to function as a filtering device and solids accumulate in the upper space. With the present apparatus the chances that these solids enter the non-broken filter elements when the apparatus is regularly backflushed are small. This is because when backflushing only the clean gas which is present between the cap and the filter outlet conduit of the non-broken filter element(s) will be entrained into the xe2x80x98cleanxe2x80x99 or downstream side of the non-broken filter element.